The present invention relates to a method of installing an underwater riser for the offshore transportation of hydrocarbons.
More specifically, the invention relates to the installation of risers for extracting and carrying gas or crude oil from a seabed installation connected to submerged well heads, to then lead this gas or crude oil up to a surface installation. These risers comprise at least one vertical pipe connecting the sea bed and the surface, and referred to by its English name “riser”, and also a float connected to the top of the pipe to suspend it vertically subsurface and vertically in line with the seabed installation. The pipe has a connecting end intended to be connected to the seabed installation. Furthermore, the top of the pipe is extended by a flexible pipe which leads to the surface installation hanging as a catenary curve under its own weight. Such an installation has already been described in document FR 2 791 316.
There is then the problem of installing these risers. The problem is that subsea oil fields are exploited at increasingly deep depths and it then becomes awkward to install these risers from the water surface, and particularly to guide the connecting end toward the seabed installation in order to connect the two together.
Thus has been conceived the idea of installing a male connection element at the connecting end of the pipe and of anchoring into the sea bed a substantially frustoconical female connection element in which the male connection element can engage. Further, pulling cables are attached to the connecting end and run as far as the female connection element to enter return pulleys. Next, these cables run up to the surface, where they can then be driven via a surface ship. Hence, from the surface, driving the pulling cables through the return pulleys, the connecting end is thereby driven, in the opposite direction, toward the seabed installation and, more specifically, as it nears the bottom, the male element is driven into the female connection element.
Reference may be made to document FR 2 791 316 the subject of which is specifically a method of installing a subsea riser of this type.
However, when the male element engages in the female element in order to make the connection between the connecting end and the seabed installation, it is necessary to pull very slowly and very carefully on the cables from the surface in order not to drive the male element violently into the female element and damage the connecting end. However, the surface ship is often subject to the influence of the waves and currents, or even the wind, which means that the male element is driven toward the female element in an oscillatory manner and therefore strikes it when they are close enough together. This phenomenon, commonly known as “pounding”, plays a part in damaging the connecting end.
Another disadvantage lies in a step prior to connecting the connecting end and said seabed installation and whereby said float and said pipe are held by a suspension line from a surface vessel. The float is filled with water so that it can be submerged with the pipe and is held between the bottom and the surface, suspended from the suspension line, which bears tensile forces corresponding to the weight of said float and of said pipe. The connecting end of the pipe is then anchored to the sea bed by moorings some distance from the seabed installation. Next, the float is unballasted so that it takes up the tensile forces corresponding to its own weight and that of said pipe and so as to keep the pipe in a vertical position. However, as soon as the intensity of the force experienced by the suspension line is taken up by the moorings, the vertical movements of the surface vessel which are caused by the waves lead to considerable tensile forces in the assembly extending from the suspension line to the moorings which particularly carry the risk of failing. Furthermore, under certain conditions, the opposing movements of the surface vessel and of the float may also, given their respective inertias, cause the surface vessel to become detached from the float.